This invention relates generally to rock drills and more particularly to a rock drill bit. The invention is also directed to a method of securing a removable head on the shank of a bit.
All rock drill bits comprise two parts--a head and a shank--which may or may not be integral. The function of the head is to break rock and to direct bits of broken rock away from the work face. Typically, tungsten carbide buttons or chisels permanently inserted into the working face of the head perform the breaking function. The head material must be tough and wear-resistant to withstand continual hammering and the abrasive effects of broken rock. Suitable head materials include through-hardening tool steels such as A2, A8 and D2; through-hardening alloy steel containing at least 1.8% nickel; work-hardening steels such as Hadfield steel, or case-hardening steels containing 0.15% to 0.25% carbon and at least 1.8% nickel. These materials have preferred characteristics, but other materials may be used instead. The shank's function is primarily to transfer percussive energy from a hammer to the head of the bit. The bit is also rotated in use, so some torque may be developed as well. Suitable materials for the shank may contain 1.8% or more nickel.
If the head and the shank of a bit can be made separately, then the best material for each may be selected, based on technical and economic requirements. Furthermore, the diameter of stock material used to make each component may be selected for that component alone, reducing scrap and costs. It remains to join the head to the shank in a way that retains the head with enough force so that is does not come off the shank in use, while permitting the head to be removed without damaging either part, particularly the shank. This of course rules out welding and other permanent bonding methods. If the components are designed for a press fit, the resulting union is not reliable under the hammering the bit takes in use. Furthermore, heavy press fits would damage the surfaces of the components, possibly rendering the shank unsuitable for re-use.
We have found that shrink fitting is a particularly suitable method of assembly, particularly when the parts have non-uniform cross-sections, and have developed methods for assembling and disassembling drill bit components having a 0.15% interference fit.